A method and a system for separating an image signal into a set of image planes. The system comprises a min-max module, a dynamic threshold module and a separation module. The min-max module receives the image signal, searches for a minimum and maximum within at least one window centered on a current pixel in the image signal. The dynamic threshold module computes, for the window, based on the respective minimum and maximum received from the min-max module and the current pixel, a respective indicator representing the distance and direction of the current pixel relative to a respective threshold plane, and outputs a control signal based on the indicator. The separation module separates the image signal into the set of image planes in accordance with the control signal by including a representation of the current pixel in at least one of the image planes.